An intact human foot, connected to its ankle, travels through stance and swing phases of a gait cycle during each stride of motion, whether the motion involves walking, jogging, or running. By adjusting the stiffness and damping characteristics of a prosthetic ankle or knee mechanism, the springiness of the intact natural human foot and its corresponding natural human joints may be mimicked, thereby optimizing the prosthetic for the desired motion of the wearer. The characteristics that are desired to store and release energy appropriately for walking tend to oppose those best suited to fast walking and running.
In a stance phase, the foot is in contact with the ground and the weight of a person is supported on the foot. In a swing phase, the foot is off the ground as the entire leg and foot move from a posterior position to an anterior position with respect to a center of gravity of the person.
The stance phase A, as shown in FIG. 1, begins just after completion of the swing phase and commences with a heel strike wherein the foot is lowered to the ground as the body moves forward from a position posterior to the person's center of gravity. Immediately after heel strike, the foot moves from a dorsi-flexed position, wherein the toes of the foot are pointed upwards, to a plantar-flexed position B wherein the bottom of the foot or shoe is flat on the walking surface, which provides greater stability as the entire weight of the person is shifted over the foot in contact with the ground.
The swing phase C commences just after heel strike of the other foot. During the swing phase, the foot is again in the dorsi-flexed position D as the foot leaves the walking surface and the foot and leg swing forward in preparation for the stance phase. Dorsi-flexion is important for normal human locomotion, since the toes are dorsi-flexed in order to clear the floor. If the foot is not dorsi-flexed during the swing phase, it would most likely catch on the walking surface and cause the person to stumble and fall, which may lead to serious injury.
It is beneficial for the joint mechanism of a prosthetic ankle to have the ability to resist plantar flexion at heel strike and to store energy during dorsi flexion/extension. During the swing phase, lifting the toe is also desirable. If the dynamic response is too stiff, the foot bounces back too quickly. If the spring is too soft, it stores less energy and releases too late. Similar considerations apply in the case of such a prosthetic joint acting in the role of a knee, and the mechanical coupling of these two joints acting in combination complicates the creation of a solution which is satisfactory throughout the range of motion.
There are prosthetic ankle joints currently available which are designed to assist a user during walking or travel through stance and swing phases of a gait cycle. Prior prosthetic devices have difficulty efficiently accommodating a combination of slopes in sideward fashion that act upon the bottom of the foot during travel by an amputee. The movements of the human foot are so complex, that even sophisticated prosthetic devices do not imitate many important aspects of the human ankle movement during walking. In this respect, the available prosthetic ankle joints are not fully capable of movement from side to side, which is needed to encompass the required range of movements of the human ankle in inversion and eversion positions accompanied by the controlled return thereof to the medial or neutral position.
There are known multi-axial ankle joints seeking to mimic those aforementioned actions of the ankle as the foot comes into contact with the ground surface attaining the instantaneous inversion/eversion and medial/neutral geometries through the motion. However, such devices do not capitalize on certain dynamic characteristics of the motion of the ankle joint in combination with the knee joint during the periods of plantar flexion and dorsi-flexion.
Other prior art joints do not adequately address the problems amputees face in getting lower limb prostheses to behave more like the original equipment. Simplistic joint designs exhibit a simplistic relationship between external influence and behavior of the knee joint. Specifically, while the joint is unloaded over the swing phase, the joint maintains itself in a locked status, and conversely, when loaded on stance phase initiation, it toggles to unlocked mode. This bi-polar mode shift regimen of control does not effectively serve a user's range of possible needs across different gait scenarios. Moreover, the absence of a regulated impedance aspect as part of the control regimen for this design fails to address such persistent issues as the tendency of the lower limb to essentially kick the user's posterior at the culmination of swing phase, and the impact suffered at the terminal point of each swing phase in gait.
Some knee-joint focused designs rely on a toe-loading moment to exceed a given threshold level before the knee joint is enabled to go into locked mode. This means initiation is subject to a potential of premature or latent knee joint mode shift under certain exigencies of the terrain being navigated by a user.
The action of a human knee joint requires less mechanism complexity in replication than that of the ankle, because it is essentially a hinge, but coordinating its dynamic response characteristics to suitably match those of the ankle joint, over the range of intended cadence is challenging. The knee mechanism should have the ability to exhibit high resistance to bending and compliance during stance phase. Simultaneously, the mechanism will desirably dissipate energy and store then release energy.
Furthermore, there are several other problems limiting the success of attempts in producing lower limb prosthetic devices. Maintaining compactness of the ankle joint to match dimensions of variably sized shoes has traditionally been a design obstacle. Similarly, issues surrounding the relationship between mechanical stress generated as a result of shear force and bending acting along the principal hinge axis exceeding material strength limits create engineering challenges. In addition, there is the need to minimize component wall thicknesses and lateral dimensions across the joint.